階上

1. Origins, structure, and basic meaning

• Structure: 階上(かいじょう) is a noun for upstairs or an upper floor relative to the present level.
• Basic usage: The source of a sound appears in 階上から音が聞こえる, while a room on that floor is 階上の空き部屋.

2. Common pitfalls and similar expressions

• Direction and floor
  • It focuses on a higher floor or its space within a building, not merely an unspecified upward direction.
• Source and modifier
  • Use 階上から for the source of a sound and 階上の for a room belonging to the upper floor.